I went down the Cucumber, Rpsec, Webrat root for the very reason that it's well documented in the Beta version of the RSpec book. I'd initially chucked myself in the deep end by trying to set all this up with Rails 3 and I'm now glad I reverted to 2.3.5. The gem developers are busy chasing the Rails 3 tail to document anything properly.
It looks like there are other contenders like Capybara instead of Webrat but I figured I'd go with the better documented but potentially inferior setup.
I also purchased User Stories applied by Mike Cohn that seems to be an inspiration for many of the ideas behind RSpec and Cucumber and is frequently referenced in the RSpec book. It had some good Amazon reviews and I've since found it to be a good buy.
Pivotal Tracker has been a great help in turning the theory into practice as well.
Just my thoughts as a fellow Rails BDD noob.